Consuming OData Services in SAP CAP

When building CAP applications that need to talk to an external OData services, one of the first things you need to solve is dealing with the schema. You can use cds import to help with this.

Given an OData metadata document (a $metadata XML file), cds import converts it into a .cds file for you to work with in CAP:

cds import ./MendixService.xml --as cds --out srv/external

You can also pass it the $metadata URL directly:

cds import 'https://user:pass@some/site/$metadata' --as cds --out srv/external

Once imported, you can use the service in your CAP project.

Remote OData Provider: Mendix

I used Mendix to quickly expose an OData service. We start by setting up a simple test entity:

Test Entity

With this in place, we set up an OData service as follows:

Entity Exposed through OData

We set the service up for OData-4 with Basic Authentication:

OData Settings

Running the project will provide the OData service and $metadata endpoint.

On the CAP side, the goal is to expose this entity through a CAP service and delegate all reads and writes to Mendix.

Consuming the Service From CAP

Start by getting the metadata from the remote service:

#!/bin/bash
USER=MxAdmin
PASS=1
curl -s -u "$USER:$PASS" http://localhost:8080/odata/data/\$metadata > MendixService.xml

Next we import the metadata using cd importas follows:

cds import ./MendixService.xml --as cds --out srv/external

This generates srv/external.cds , which should look something like the following:

/* checksum : afc6b7830464a3fbaa91f0aa7cbf29a2 */
@cds.external : true
@Capabilities.BatchSupported : false
@Capabilities.CrossJoinSupported : false
@Capabilities.QuerySegmentSupported : true
@Capabilities.SupportedFormats : [ 'application/json' ]
service MendixService {
  @cds.external : true
  @cds.persistence.skip : true
  entity Data {
    key ID : Integer64 not null;
    x : Integer64;
    y : Integer64;
  };
};

Next, ensure the following in package.json:

"cds": {
  "requires": {
    "MendixService": {
      "kind": "odata-v4",
      "model": "srv/external.cds",
      "credentials": {
        "url": "http://localhosto/odata/data"
      }
    }
  }
}

For the username and password, you can add that in .cdsrc-private.json:

{
  "requires": {
    "MendixService": {
      "credentials": {
        "username": "myuser",
        "password": "mypassword"
      }
    }
  }
}

Ensure .cdsrc-private.json is added to your .gitignore

Define Service

Next, define a service at srv/data-service.cds:

service DataService {
  entity Data {
    key ID : Integer64 not null;
    x     : Integer;
    y     : Integer;
  }
}

Implement Service

Create srv/data-service.js. For READ, UPDATE, and DELETE, CAP's remote service adapter handles query translation cleanly.

const cds = require('@sap/cds');

module.exports = cds.service.impl(async function (srv) {

  const mendix = await cds.connect.to('MendixService');
  const { Data } = mendix.entities;

  srv.on('READ', 'Data', async (req) => {
    return mendix.run(SELECT.from(Data));
  });

  srv.on('CREATE', 'Data', async (req) => {
    return mendix.run(INSERT.into(Data).entries(req.data));
  });

  srv.on('UPDATE', 'Data', async (req) => {
    const { ID } = req.data;
    await mendix.run(UPDATE(Data).set(req.data).where({ ID }));
    return mendix.run(SELECT.one.from(Data).where({ ID }));
  });

  srv.on('DELETE', 'Data', async (req) => {
    const { ID } = req.params[0];
    return mendix.run(DELETE.from(Data).where({ ID }));
  });

});

You can now run the server using cds watch

Testing

You can use the following OData requests for testing:

# READ
curl "http://localhost:4004/odata/v4/data/Data"

# CREATE
curl -X POST "http://localhost:4004/odata/v4/data/Data" \
  -H "Content-Type: application/json" \
  -d '{"x": 1, "y": 2}'

# UPDATE
curl -X PATCH "http://localhost:4004/odata/v4/data/Data(10)" \
  -H "Content-Type: application/json" \
  -d '{"x": 99}'

# DELETE
curl -X DELETE "http://localhost:4004/odata/v4/data/Data(10)"

With logging enable on the remote server, you should be able to verify and debug the remote calls:

Mendix Server Logs

References

• CAP: Consume External Service